An ophthalmic examination should not be a scary experience! Though admittedly interpretation of the findings may sometimes be challenging, the examination itself follows a logical, anatomical order. Furthermore, it does not require expensive equipment. In fact, the most important items required are non-ophthalmic in nature: a room that can be darkened, a good source of focal light and a magnifying loupe. A hand held lens, a direct ophthalmoscope, a tonometer and some disposable items (stains, solutions, etc.) complete the list of basic equipment.

Gross Inspection

The patient should be observed as it walks into the room, since this is an unfamiliar environment which may highlight visual deficits; these will be further evaluated later on. Following the anamnesis and physical examination, the ocular assessment begins by careful observation of the patient from a distance, without touching the patient (as this may cause distortion of palpebral fissure). While observing, ask yourself:

 Are both eyes open normally? Is there evidence of pain or photophobia? Is the animal blinking normally?

 Are the eyes of normal size and position? Is there evidence of exophthalmos or buphthalmos? Are the pupils of equal size?

 Is the eyelid conformation normal? Is there evidence of entropion or ectropion (usually of the lower lid)? Is the 3rd eyelid elevated?

 Is there ocular discharge? What is its nature?

Assessing Vision

Vision is assessed primarily by evaluating the menace response, in conjunction with testing the papillary light response (PLR). These tests are discussed below ("Assessment and Examination of Blind Patients").¹

Examination in the Dark

After the light has been dimmed, the dilatation of the pupils should be evaluated. Use a dim light (to prevent constriction), and stand at a distance so you can visualize both pupils simultaneously, using the tapetal reflection. The tapetal reflection also serves to highlight any ocular opacities, particularly in the lens or vitreous. Next, use a bright light to evaluate the Pupillary Light Reflex (PLR), discussed below.

Magnification is required for the next stages of the examination. Once again, the lid margins, conjunctiva and corneal surface are examined. Use the magnification to check for aberrant eyelashes (trichiasis, distichia); these can be best visualized against the white background of the conjunctiva, by slightly pulling the eyelid. Following the anatomical order, next inspect the anterior chamber (looking for opacities in the aqueous), the iris surface and the anterior segment of the lens.

Ophthalmoscopy

Ophthalmoscopy should be conducted in a dark room, following dilatation of the pupil. Carefully inspect the entire fundus, evaluating changes in the tapetum, non-tapetum, blood vessels and optic disc. It is best to stay in stationary position and let the patient's eye movements bring the structures to you, instead of trying to "chase" them.²

Additional Tests

 Schirmer tear test is used to evaluate tear production and diagnose keratoconjunctivitis sicca.

 Fluorescein staining is used to diagnose corneal ulcers. Superficial ulcers may be stained with Rose Bengal.

 Samples for bacteriology, mycology and cytology may be taken as indicated. The first two should be taken before any drops are put in the eye, as ophthalmic solutions frequently contain preservatives.

 Nasolacrimal patency is evaluated by passage of fluorescein from the eye to the nose, by cannulating the nasolacrimal system and by dacryocystorhinography.

 Ultrasound is frequently used in ophthalmology. The main indications are imaging of the retrobulbar area, and imaging of the posterior segment when it can not be visualized.

 Tonometry-measuring IOP to diagnose glaucoma.

 Additional tests, including gonioscopy (evaluation of the iridocorneal angle as part of the diagnosis of glaucoma) and electroretinography (recording electrical responses of the retina to flashes of light, to determine retinal function) may be available in referral centers.

Assessment and Examination of Blind Patients

History

As with any patient, you should begin by taking a history. Inquire whether the blindness is acute, or of gradual onset. Inquire whether the blindness was associated with preferential loss of night vision. One of the first behavioral signs of inherited, degenerative diseases of the outer retina (commonly known as Progressive Retinal Atrophy, PRA) is loss of night vision as rods are affected before cones. Finally, inquire whether the animal is healthy, or are there other signs of illness, besides loss of vision? Blindness may be caused by numerous systemic or neurological diseases.^1,3

Assessing the Visual System in the Blind Animal

Menace Response

This involves making a sudden threatening gesture which is supposed to elicit a blink response. It is important to note that the menace response involves cerebral cortical integration and interpretation and therefore is not a reflex. Rather, it is a cortical response that requires the entire peripheral and central visual pathways, as well as the visual cortex and the facial nucleus of cranial nerve VII, to be intact.^2,4

To avoid false positive responses from the visual, contralateral eye, the menace response should be evaluated in one eye, while the other eye is covered. Be careful not to touch the eyelashes/hair of the patient, or cause air movement, as these may also elicit false positive response. False negative responses may be caused by facial nerve paralysis. Therefore, in the absence of a menace response always test the blinking reflex by touching of the skin at the canthus. Remember that the menace response is absent in very young (< 10–12 weeks) animals, and may also be affected by the mental state of the patient.^1,4

Additional Visual Tests

Vision can be assessed using an obstacle course. The course may be navigated in light and dim environments, and with one eye patched. Be consistent in the obstacle course that you construct, and make sure it can be navigated by normal animals!

The visual placing response is useful when results of the obstacle course and menace response are equivocal. Lift the animal towards the table, allowing it to see the approaching surface. A normal animal will extend its leg towards the surface before its paw touches the table.

The Pupillary Light Reflex (PLR) & Dazzle Reflex

Unlike the menace response, the PLR is a subcortical reflex.

Therefore, it does NOT test vision, and may be normal in a cortically blind animal. Furthermore, the PLR is usually present (though it may be diminished or slow) in animals suffering from PRA, cataracts, and other causes of subcortical blindness.

The dazzle reflex is another subcortical reflex. It is manifested as substitute for the PLR in cases when the pupils can't be seen, such as in cases of severe corneal edema or hyphema.

Localizing the Lesion in the Blind Patient

Based on the results of the ophthalmic examination, the patient may be categorized into one of 4 general categories:

1.  Abnormal ophthalmic findings combined with a normal/diminished PLR. Patients in this category suffer either from opacity of the ocular media (e.g., corneal edema, hyphema/hypopyon, cataract, etc.) or from retinal disease (e.g., PRA).

2.  Abnormal ophthalmic examination and an absence of PLR. Common diseases in this category include glaucoma, retinal detachment and optic neuritis involving the proximal optic nerve.

3.  Normal ophthalmic examination and an absence of PLR. Common diseases in this category include Sudden Acquired Retinal Degeneration (SARD), optic neuritis involving the distal optic nerve, and neoplasia compressing the optic nerve or chiasm.

4.  Normal ophthalmic examination and absence of PLR. These are usually neurological cases, caused by central lesions affecting the visual pathways from the lateral geniculate nucleus to the contralateral visual cortex.^1,3,4

References

1.  Maggs DJ. Basic diagnostic techniques. In: Maggs DJ, Miller PE, Ofri R, eds. Slatter's Fundamentals of Veterinary Ophthalmology, 4th edition. St Louis: Saunders Elsevier, 2007:81–106.

2.  Barnett KC, Heinrich C, Sansom J. Examination of the eye and adnexa. In: Barnett KC, Heinrich C, Sansom J, eds. Canine Ophthalmology. An Atlas & Text. London: WB Saunders, 2002:1–8.

3.  Stades FC, Wyman M, Boeve MH, Neumann W, Spiess BM. Examination of the eye and its adnexa. In: Stades FC, Wyman M, Boeve MH, Neumann W, Spiess BM, eds. Ophthalmology for the Veterinary Practitioner, 2nd ed. Hannover: Schlutersche-Verlagsgesellschaft, 2007:31–46.

4.  Gelatt KN. Ophthalmic examination and diagnostic procedures. In: Gelatt KN, ed. Essentials of Veterinary Ophthalmology. Philadelphia: Lippincott Williams & Wilkins, 2000:1–26.

5.  Ollivier FJ, et al. The eye examination and diagnostic procedures. In: Gelatt KN, ed. Veterinary Ophthalmology, 4th ed. Ames, Iowa: Blackwell Publishing, 2007:438–483.